Ruling on the prayer of one who leaves the first tashahhud, whether out of forgetfulness or intentionally

Q: If the imam completes the prayer and stands up without performing the first tashahhud, and is reminded but does not return, and when he concludes the prayer with the taslim he is reminded but does not prostrate, and after the taslim some of the ma'mum (those praying behind him) ask him: "Why did you not prostrate?" and he says: "Its time has passed," what is required of him? Does he have to repeat the prayer because he intentionally left an obligatory act? And if he was ignorant and the ma'mum were also ignorant of the ruling, what is required of them? Please issue a ruling for us, may Allah reward you.

A: If the one praying intentionally leaves the first tashahhud, his prayer is invalid according to the most correct of the two opinions of the scholars, provided he is aware of the ruling and is in a state of remembrance. If he is ignorant, he owes nothing. If he leaves it out of forgetfulness, it is obligatory for him to perform the prostration for forgetfulness. If he intentionally leaves it, his prayer is invalid. However, if he forgets and concludes the prayer with the taslim before prostrating, and then is reminded or remembers, it is obligatory for him to prostrate for forgetfulness after the taslim, and then to conclude the prayer with the taslim, just as in the case of the prostration for forgetfulness whose place is after the taslim. If he does not do so, there is a difference of opinion regarding the invalidity of the prayer due to this, meaning due to leaving the prostration for forgetfulness after the taslim, whether its place is after the taslim or before it, and he forgot it so that it became after the taslim.
Abu Muhammad ibn Qudamah (may Allah have mercy on him) said in al-Mughni: "If he intentionally leaves an obligatory act, if it is before the taslim, his prayer is invalid because he neglected an obligatory act in the prayer intentionally. If he leaves an obligatory act after the taslim, his prayer is not invalid; because it is a means of completing the worship outside of it, so it is not invalidated by leaving it, just as the means of completing Hajj are not invalidated by leaving them. It is the same whether its place is after the taslim or before it, and he forgot it so that it became after the taslim. It has been reported from Ahmad what indicates the invalidity of the prayer, and it has been reported from him that he suspended judgment." This is the end of the intended quote.
And by this it is known that the correct view is that the prayer is valid and that repeating it is not obligatory for everyone, except if the imam intentionally left what is prescribed to be done before the taslim while being aware of the legal ruling. In that case, he is obligated to repeat it because he left an obligatory act without a legal excuse. As for the ma'mum, it is obligatory for him to prostrate for forgetfulness if his imam did not prostrate after the taslim, according to the opinion of the majority, as stated in al-Mughni; because forgetfulness diminishes the prayer of everyone. So if the imam does not prostrate to make up for the deficiency caused by forgetfulness, it is obligatory for the ma'mum to prostrate, whether they prostrate individually or appoint someone to lead them in it; because when the imam refused the obligatory act, their following of him was severed, and it became obligatory for them to independently perform the obligatory act. Just as if he concluded the prayer with a deficiency and they reminded him but he did not return to the correct state, it is obligatory for them to complete their prayer individually or with an imam from among them, because completing the prayer is obligatory for everyone. So when the imam refused it, their following of him was severed. If they do not prostrate, their prayer is not invalid; because it is an obligatory act outside the prayer, so the prayer is not invalidated by leaving it, just as the adhan and iqamah are not invalidated by leaving them, and just as the means of completing Hajj are not invalidated by leaving them. And Allah the Exalted knows best.